Piezoelectric materials such as PZT (lead zirconate titanate) have conventionally been used as electromechanical converting elements such as drive elements and sensors. On the other hand, a MEMS (Micro Electro Mechanical Systems) element using a Si substrate is increasing in number in response to recent years' requests for downsizing, densification, further cost reduction, and the like of an apparatus. If the piezoelectric material is applied to the MEMS element, various devices such as inkjet heads, ultrasonic sensors, infrared sensors, and frequency filters can be created.
Here, if the piezoelectric material is applied to the MEMS element, it is desired to make the piezoelectric material into thin film. This is because making the piezoelectric material into thin film has the following advantages: In other words, highly accurate processing using semiconductor processing techniques such as film deposition and photolithography becomes possible, and downsizing and densification can be achieved. The piezoelectric material can be processed over a large-area wafer in one batch. Accordingly, the cost can be reduced. The conversion efficiency of an electric machine improves, and the properties of the drive element and the sensitivity of the sensor improve.
Chemical methods such as CVD (Chemical Vapor Deposition), physical methods such as sputtering and ion plating, and deposition methods in a liquid phase such as the sol-gel process are known as a method for depositing the piezoelectric material on a substrate of Si or the like.
The piezoelectric material such as PZT is generally an ABO3-type oxide, which is known to show an excellent piezoelectric effect when its crystal takes a perovskite structure. FIG. 10 schematically illustrates a crystal structure of PZT. For example, in a tetragon of Pb(Zrx, Ti1-x)O3, the perovskite structure is a structure where a Pb atom is located at vertexes (A sites) of the tetragon, a Ti or Zr atom is located at a body center (B site), and an O atom is located at face centers.
Moreover, PZT is a solid solution of PTO (PbTiO3; titanate) and PZO (PbZrO3; zirconate), which both take the perovskite structure. The entire PZT becomes tetragonal when the ratio of PTO is high, and becomes rhombohedral when the ratio of PZO is high.
FIG. 11 illustrates the relationship between the composition ratio of PTO to PZO and the crystal system. When the composition ratio of PTO to PZO is approximately 48/52 to 47/53, the crystal system changes from a tetragon to a rhombohedron, or from a rhombohedron to a tetragon. A boundary where the crystal system changes in this manner is referred to as the morphotropic phase boundary (MPB; Morphotropic phase boundary), and hereinafter also simply referred to as the phase boundary. Around room temperature, the crystal structure of PZT is tetragonal, rhombohedral, or a mixed crystal (phase boundary) of them. However, at or above the Curie temperature, the crystal structure of PZT becomes a cubit crystal at any composition ratio of PTO to PZO.
FIG. 12 illustrates the relationship between the composition ratio of PTO to PZO and the properties (relative permittivity and electromechanical coupling factor). On the above phase boundary, both the relative permittivity and the electromechanical coupling factor specifically increase. There is a positive correlation between the relative permittivity and a piezoelectric constant (a displacement per unit electric field). When the relative permittivity increases, then the piezoelectric constant increases. Moreover, the electromechanical coupling factor is an indicator of efficiency of the conversion from an electrical signal into mechanical distortion, or from mechanical distortion into an electrical signal. When the factor increases, the conversion efficiency increases.
If PZT is formed into thin film (of a thickness of approximately several microns) on a Si substrate, it is difficult to obtain desired properties. This is considered because the perovskite structure required in PZT, and the above phase boundary cannot be achieved due to residual stress caused by differences in lattice constant and linear expansion coefficient between the substrate and lower electrode, and PZT. If the crystallinity of PZT (especially an initial layer) in the vicinity of the interface with the substrate and the lower electrode decreases due to the influence of such differences in lattice constant and linear expansion coefficient, reliability such as the adhesion between the piezoelectric layer and the substrate, and dielectric strength significantly decreases.
Hence, a technology is known which provides an underlying layer (buffer layer or seed layer) for controlling the crystallinity of the piezoelectric layer between the substrate and the piezoelectric layer. For example, in Patent Literature 1, an underlying layer including PLT (lead lanthanum titanate) is provided between a substrate and a piezoelectric layer (for example, PLZT; lead lanthanum zirconate titanate). PLT of the underlying layer has a property where a perovskite crystal tends to occur even on a Si substrate and a lower electrode. Therefore, it becomes easy to form the piezoelectric layer with the perovskite structure by forming the piezoelectric layer on such an underlying layer.
However, the composition of the underlying layer (PLT) is not in perfect agreement with that of the piezoelectric layer (PLZT). Accordingly, even if PLT is used as the underlying layer, the disagreement between the underlying layer and the piezoelectric layer cannot be perfectly solved. As a result, a low region with low crystallinity still remains in the initial layer of the piezoelectric layer.
Hence, for example, in Patent Literature 2, a middle layer including the gradient of the composition that has continuously been changed to the composition in the vicinity of the phase boundary of PZT by continuously increasing the concentration of Zr (zirconium) is provided between a first layer including PLT and a second layer including PZT, and the disagreements of the lattice constant and the linear expansion coefficient are eased in the middle layer.